This invention relates to an oxygen sensing element capable of measuring partial pressures of oxygen in sample gases. More particularly, it relates to an oxygen sensing element suitable for use in an exhaust gas purifying system wherein the content of oxygen in an exhaust gas from an automobile internal combustion engine is measured, thereby to determine the content of unburnt hydrocarbons, carbon monoxide and nitrogen oxides in the exhaust gas and, based on the measurement results, the air-fuel ratio is appropriately adjusted so that the efficiency of a catalyst, for purifying the exhaust gas is enhanced; or suitable for use in a device for measuring the concentration of oxygen in a molten metal in the course of metal refining.
An oxygen sensor is an oxygen concentration cell having a structure such that electrodes are mounted on the opposite sides of a solid electrolyte composed of a sintered ceramic material capable of conducting an oxygen ion. An electromotive force is produced across the solid electrolyte by the difference between the partial pressures of oxygen in reference and sample gases contacting opposite sides of the solid electrolyte. The concentration of oxygen in the sample gas can be determined by measuring the electromotive force so produced. That is, as is well known, assuming that the partial pressures of oxygen in the reference and sample gases are PO.sub.2 (1) and PO.sub.2 (2), respectively, the electromotive force E produced between the electrodes on the opposite sides of the solid electrolyte is expressed by the following equation. ##EQU1## wherein R is gas constant, T is absolute temperature and F is Faraday's constant. Thus, if the partial pressure of oxygen PO.sub.2 (1) in the reference gas is known, the partial pressure of oxygen PO.sub.2 (2) in the sample gas can be determined from the above-mentioned equation by measuring the electromotive force E. Conventionally, air is used as the reference gas. The reference gas may also be generated chemically by using a mixture of a metal and its oxide which produces an equilibrium partial pressure of oxygen. This reference gas-generating metal-metal oxide mixture is hereinafter referred to as "reference solid electrode" for brevity.
However, the conventional oxygen sensors, wherein the reference solid electrode of a metal-metal oxide mixture is employed, are not advantageous compared with the oxygen sensors wherein air is used as the reference gas. This is because the former oxygen sensors do not successfully operate at a low temperature. That is, at a temperature lower than about 400.degree. C., the former oxygen sensors generate little or no electromotive force and the internal impedance thereof is undesirably increased together with an apparent reduction of the electromotive force. In order to overcome this defect, it has been proposed to provide an electrode layer on the interface between the metal-metal oxide mixture reference solid electrode and the solid electrolyte, which electrode layer is composed of an electrochemically active metal such as platinum. The electrode layer accelerates the conversion of oxygen ions to molecular or atomic oxygen according to the following formula and, thus, reduces the polarization occurring in the metal-metal oxide mixture. EQU 20.sup.-- --O.sub.2 (or 20)+4e.sup.-
Such an electrode layer is formed by chemical or electrical plating, ion-plating or the like. However, the formation of such an electrode layer is complicated, and it is difficult to avoid a variability of some performances such as the operating temperature, the response time and the internal resistance among the resulting oxygen sensors.
Japanese Patent Publication (KOKAI) No. 9497/1976 discloses an oxygen sensing electrochemical cell having a structure such that a reference medium of a metal-metal oxide mixture is completely enclosed within a solid electrolyte member having an electrode mounted on the exterior surface thereof. This oxygen sensing electrochemical cell does not have such a defect as is encountered in the above-metioned oxygen sensing cell provided with an electrochemically active metal electrode layer on the interface between the metal-metal oxide mixture reference solid electrode and the solid electrolyte. This cell is, however, still not satisfactory in its operability at a low temperature.
In addition, the conventional oxygen sensors, which are composed of a reference solid electrode, a solid electrolyte and, optionally, an electrochemically active metal electrode layer on the interface between the reference solid electrode and the solid electrolyte, have the following defects. That is, these oxygen sensors are liable to be distorted or cracked, and on occasion the metal electrode layer is separated from the reference solid electrode and/or the solid electrolyte, during the high temperature operation of these sensors or during the step of sintering these sensors in the course of their manufacture. These defects lead to a reduction in the responsiveness of the oxygen sensors.